PRTN3 and rheumatoid arthritis: For example, HLA-DRB1 alleles encoding the shared epitope a “shared” motif (QKRAA, QRRAA or RRRAA in positions 70–74 of the DRB1 chain) that is found on DR1 and DR4 alleles associated with a distinct subset of individuals with RA, specifically those who have anti-citrulline antibodies (ACPA) or ACPA+ RA (reviewed in (24)).